Optical transmission line

ABSTRACT

An optical transmission line including a portion formed by fusion-splicing optical fibers having structures different from each other; wherein, in the optical fibers having structures different from each other, a first optical fiber 1 has a mode field diameter smaller than that of a second optical fiber 2 fusion-spliced thereto; and wherein the first optical fiber 1 has an average viscosity from a center to an outermost layer greater than that of the second optical fiber from a center to an outermost layer.

TECHNICAL FIELD

The present invention relates to an optical transmission line composedof fusion-splicing optical fibers which have different structures eachother; and, in particular, to an optical transmission line including aportion in which optical fibers having mode field diameters differentfrom each other are fusion-spliced.

BACKGROUND ART

Optical fibers are connected together by fusion splice, which enables apermanent connection, in order to restrain the splice loss at theirsplice portion from fluctuating. However, the splice loss at thefusion-splice portion is greater when optical fibers having structuresdifferent from each other are fusion-spliced together than when opticalfibers having the same structure are fusion-spliced together

For example, there is a case where a dispersion-compensating opticalfiber having a negative chromatic dispersion at a wavelength of 1.55 μmis fusion-spliced to a standard single-mode optical fiber having azero-dispersion wavelength in a 1.3-μm wavelength band and a positivechromatic dispersion at a wavelength of 1.55 μm, so as to carry outdispersion compensation. The single-mode optical fiber anddispersion-compensating optical fiber greatly differ from each other interms of their fiber structures. Therefore, the splice loss at theirfusion-splice portion is about 1.0 to 2.0 dB, which is large.

Constructing an optical transmission line by alternately fusion-splicepositive and negative dispersion optical fibers respectively havingpositive and negative chromatic dispersions at a predeterminedwavelength, for example, has also been under consideration. Constructingan optical transmission line as such yields a predetermined value ofchromatic dispersion or higher at each point on the optical transmissionline, so as to restrain transmission characteristics from deterioratingdue to four-wave mixing, and lowers the average chromatic dispersion ofthe optical transmission line as a whole, so as to restrain transmissioncharacteristics from deteriorating due to the chromatic dispersion. Inthis case, for example, the positive dispersion optical fiber has astep-index type refractive index profile with a core diameter of 8 μmand a refractive index difference of 0.35%, whereas the negativedispersion optical fiber has a W type refractive index profile, wherebytheir fiber structures greatly differ from each other. Therefore, thesplice loss at their fusion-splice portion is about 0.8 to 1.5 dB, whichis large.

Optical fiber connecting methods for eliminating such problems aredisclosed in Japanese Patent Application Laid-Open No. HEI 3-130705 andJapanese Patent Application Laid-Open No. SHO 57-24906. In the opticalfiber connecting method disclosed in Japanese Patent ApplicationLaid-Open No. HEI 3-130705, a first optical fiber having a larger corediameter and a smaller relative refractive index difference and a secondoptical fiber having a smaller core diameter and a greater relativerefractive index difference are fusion-spliced together, and thusfusion-splice portion is heat-treated at a predetermined temperature thereafter. In the optical fiber connecting method disclosed in JapanesePatent Application Laid-Open No. SHO 57-24906, on the other hand, thefirst optical fiber whose core region has a higher refractive index isheat-treated more strongly than the second optical fiber after fusionsplice. Both of the methods intend to diffuse dopants in any of thefirst and second optical fibers upon the heat treatment, so as to lowerthe difference in their core diameters, thus making it possible todecrease the splice loss at the fusion-splice portion.

Using these conventional optical fiber connecting method is supposed tolower the splice loss at the fusion-splice portion between theabove-mentioned single-mode optical fiber and dispersion-compensatingoptical fiber to about 0.3 to 0.6 dB. It is also supposed that thesplice loss at the fusion-splice portion between the above-mentionedpositive and negative dispersion optical fibers can be lowered to about0.3 dB.

DISCLOSURE OF THE INVENTION

However, the splice loss at the fusion-splice portion has not yet beenconsidered small enough although it is somewhat reduced by theconventional techniques disclosed in the above-mentioned twopublications.

The inventors of the present invention observed the glass state near thefusion-splice portion in fusion-spliced two optical fibers in detail. Asa result of the observation, it has been seen that, when a standardsingle-mode optical fiber and a dispersion-compensating optical fiberare fusion-spliced, the core region in the dispersion-compensatingoptical fiber deforms as the mode-field diameter is smaller.

Based on the inventors findings mentioned above, for eliminating theaforesaid problems, it is an object of the present invention to providean optical transmission line constituted by optical fibers havingstructures different from each other in which the connection loss attheir fusion-splice portion is further lowered.

The optical transmission line in accordance with the present inventionis an optical transmission line including a portion formed byfusion-splicing optical fibers having structures different from eachother; wherein, in the optical fibers having structures different fromeach other, a first optical fiber has a mode field diameter smaller thana mode field diameter of a second optical fiber fusion-spliced thereto;and wherein the average viscosity from the center to the outermost layerin the first optical fiber is greater than the average viscosity fromthe center to the outermost layer in the second optical fiber.

When the average viscosities in the first and second optical fibers areset as such, the deformation of the core region of the first opticalfiber having a smaller mode field diameter becomes smaller upon fusionsplice, whereby the splice loss can be restrained from increasing due tochanges in fiber structures.

Preferably, after the first and second optical fibers arefusion-spliced, the optical transmission line is heat-treated at thehighest heating temperature of at least 1300° C. but not exceeding 1800°C. within a range having a length of at least 1 mm but less than 10 mmcentered at the fusion-splice portion. In this case, the splice loss canfurther be reduced.

The first optical fiber may be one having at least two cladding regionlayers surrounding a core region, and the average viscosity of theoutermost cladding region layer greater than that of the core region. Inthis case, the cladding region does not deform upon fusion splice in thefirst optical fiber, so that the core region is restrained fromdeforming upon heating, whereby the splice loss can be kept fromincreasing. Preferably, the first optical fiber has a core region dopedwith GeO₂ at a dopant concentration of at least 18 wt %, a firstcladding region doped with F element, and an outermost cladding regionlayer doped with Cl element.

Preferably, the second optical fiber has at least one cladding regionlayer surrounding a core region, and the average viscosity of theoutermost cladding region layer lower than any of the average viscosityof the core region and that of the outermost cladding region layer inthe first optical fiber. In this case, no large structural changes occurin the core region in the second optical fiber even when its claddingsoftens upon fusion splice. Preferably, the second optical fiber has acore region doped with Cl element and a cladding region doped with Felement. Alternatively, the second optical fiber may have two claddingregion layers, the outer cladding region being doped with F element byan amount smaller than that in the inner cladding region.

Preferably, the core region of the second optical fiber has an outsidediameter greater than the inside diameter of the outermost claddingregion layer in the first optical fiber. In this case, the core regionand first cladding region in the first optical fiber greatly influencingstructural parameters thereof appear as if lidded with the core regionof the second optical fiber, thus being surrounded with glass having ahigh viscosity, whereby their forms are easier to maintain.

Preferably, a part of the cladding region in the second optical fiber isdoped with F element, whereas an outermost layer region thereof has aninside diameter of at least 1.05 times that of an outermost layer regionin the first optical fiber.

A part of the cladding region of the second optical fiber may be dopedwith F element, the average viscosity of regions inside an outermostcladding region layer is greater than three times that of a regioninside the outermost cladding region layer of the first optical fiber.Such setting can suppress the deformation of the region inside theoutermost cladding region layer in the first optical fiber, wherebyfavorable connection characteristics can be obtained.

The first and second optical fibers may have unlike sign chromaticdispersions each other. Though the first and second optical fibers havemode field diameters greatly different from each other in general, thesplice loss after fusion splice or after heat treatment can be madesmaller in this case than in the conventional cases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a longitudinal sectional view for explaining theconfiguration of a fusion-splice portion in an optical transmission linein accordance with the present invention, whereas FIG. 1B is alongitudinal sectional view for explaining the configuration, of afusion-splice portion in a conventional optical transmission line;

FIGS. 2A to 2C are views for explaining refractive index profiles ofoptical fibers used in the optical transmission lines of FIGS. 1A and1B;

FIG. 3 is a graph showing relationships between concentrations ofvarious dopants (GeO₂, Cl element, and F element) in an optical fiberand its relative refractive index difference;

FIG. 4 is a graph showing the relationship between viscosity andtemperature in each of pure silica (SiO₂) glass, silica glass doped with2 wt % of Cl element, and silica glass doped with 2 wt % of F element;

FIG. 5 is a graph showing the relationship between viscosity andtemperature in each of various dopants at 1500° C.

FIG. 6 is a view showing another embodiment of the optical transmissionline in accordance with the present invention, whereas FIG. 7 is a viewfor explaining the refractive index profile of a second optical fiberused in this embodiment; and

FIGS. 8 and 9 are graphs each showing results of a comparativeexperiment, in which FIG. 8 is a graph plotting the splice loss of eachsample after heat treatment with respect to the ratio between the corediameter of second optical fiber and the first cladding diameter offirst optical fiber, whereas FIG. 9 is a graph plotting the splice lossof each sample after heat treatment with respect to the ratio betweenthe average viscosity of the core region and first cladding region infirst optical fiber and the viscosity of core region in second opticalfiber.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, embodiments of the present invention will be explainedin detail with reference to the accompanying drawings. To facilitate thecomprehension of the explanation, the same reference numerals denote thesame parts, where possible, throughout the drawings, and a repeatedexplanation will be omitted.

FIG. 1A is a longitudinal sectional view showing a connecting portion ofdifferent kinds of optical fibers in a preferred embodiment of opticaltransmission line 5 in accordance with the present invention, whereasFIG. 1B is a longitudinal sectional view showing a splice portion of aconventional optical transmission line 6 for comparison.

Fusion-splice in the optical transmission line 5 of this embodiment arean optical fiber 1 which is a dispersion-compensating optical fiberhaving a negative chromatic dispersion at a wavelength of 1.55 μm; andan optical fiber 2 which is a single-mode optical fiber, having azero-dispersion wavelength in a 1.3-μm wavelength band and a positivechromatic dispersion at a wavelength of 1.55 μm, for an opticaltransmission line. FIGS. 2A and 2B show respective refractive indexprofiles of optical fibers 1 and 2.

As shown in FIGS. 1A and 2A, the optical fiber 1 is an optical fiberhaving a so-called double cladding, and comprises, successively from itscenter, a core region 11 having a maximum refractive index n₁₁ and anoutside diameter 2 a ₁, a first cladding region 12 having a refractiveindex n₂ and an outside diameter 2 b ₁, and a second cladding 13 havinga refractive index n₁₃, in which the individual refractive indices areset to n₁₁>n₁₃>n₁₂ in terms of the relationship of magnitude. While theoptical fiber 1 is silica glass based, the core region 11 is doped witha high concentration of germanium dioxide (GeO₂), and the first claddingregion 12 is doped with fluorine (F). The second cladding region 13 is asubstantially pure silica glass or doped with about 0.5 wt % to 1.0 wt %of chlorine (Cl). Preferably, the relative refractive index differenceΔ₁₁ of core region 11 with reference to the refractive index n₁₃ ofsecond cladding region 13 is at least 1%. FIG. 3 is a graph showingrelationships between concentrations of various dopants (GeO₂, Clelement, and F element) and relative refractive index difference, fromwhich it is seen that the dopant concentration of GeO₂ is at least 18 wt% for realizing the relative refractive index difference Δ₁₁ of coreregion 11 when the second cladding region 13 is pure silica glass.

When such setting is made, respective viscosities η₁₁, η₁₂, η₁₃ of theindividual regions 11 to 13 within the optical fiber 1 satisfy therelationship of η₁₁<η₁₂<η₁₃. FIG. 4 is a graph showing the relationshipbetween viscosity and temperature in each of pure silica (SiO₂) glass,silica glass doped with 2 wt % of Cl element, and silica glass dopedwith 2 wt % of F element. In general, as can be seen from this graph,the viscosity of silica glass decreases when any of substantially allthe other elements or oxides is added thereto. Also, its viscosity has aproperty of decreasing as the temperature rises regardless of whetherdopants exist or not. FIG. 5 is a graph showing the relationship betweendopant concentration and viscosity at 1500° C. for each of three dopantsof GeO₂, Cl, and F. At the same dopant amount (wt %), the decrease inviscosity is the largest when doped with F element, and the smallestwhen doped with GeO₂. Since the core region 11 is doped with a largeamount of GeO₂ in this embodiment, however, its viscosity η₁₁ is lowerthan the viscosity η₁₂ of the first cladding region 12 doped with Felement, whereas the viscosity η₁₃ of the second cladding region 13doped with no additive or only a minute amount of Cl element is thehighest.

On the other hand, as shown in FIGS. 1A and 2B, the optical fiber 2comprises, successively from its center, a core region 21 having amaximum refractive index n₂₁ and an outside diameter 2 a ₂, and acladding region 22 having a refractive index n₂₂, whereas the individualrefractive indices are set so as to have the relationship of n₂₁>n₂₂Theoptical fiber 2 is based on silica glass, whereas the core region 21 issubstantially pure silica glass or doped with about 0.5 wt % to 1.0 wt %of Cl element. The cladding region 22 is doped with F element. As aresult, the viscosity η₂₂ of cladding region 22 is lower than theviscosity η₂₁ of core region 21, and lower than the viscosity η₁₃ ofsecond cladding region 13 of optical fiber 1 (see FIG. 4). The opticalfiber 2 has a low transmission loss since the dopant concentration ofcore region 21 is low, and is excellent in hydrogen- andradiation-resistant characteristics since the cladding region 22 isdoped with F element, whereby it is an optical fiber suitably used forundersea cables.

Exemplified here is a case where each of the second cladding region 13of optical fiber 1 and the core region 21 of optical fiber 2 is puresilica glass, and both regions have the same refractive index level(i.e., n₁₃=n₂₁ holds). Of course, one or both of them may be doped withCl element, and refractive index levels in both regions may differ fromeach other.

Letting the respective average viscosities of optical fibers 1, 2 be η₁,η₂, the relationship of η₁>η₂ holds. Here, assuming that the opticalfiber is composed of n layers, the average viscosity η_(i) of opticalfiber i as a whole can be represented by the following expression:$\eta_{i} = {\sum\limits_{j = 1}^{n}\quad {\eta_{ij} \times \frac{S_{ij}}{S_{i}}}}$

where η_(ij) is the viscosity of its j-th layer (1≦j≦n), S_(ij) is thecross-sectional area thereof, and S_(i) is the total cross-sectionalarea.

When thus set optical fibers 1 and 2 are fusion-spliced, the respectivecore regions 11, 21 of optical fibers 1 and 2 can be restrained fromdeforming near the fusion-splice portion. This is because of the factthat the core region 11 and first cladding region 12 having a lowerviscosity in the optical fiber 1 are surrounded by the second claddingregion having a higher viscosity, and their end face at thefusion-splice portion is in a state blocked by the core region 21 havinga higher viscosity in the optical fiber 2, whereby each of them can berestrained from deforming.

When thus set optical fibers 1 and 2, which are adispersion-compensating optical fiber and a single-mode optical fiber,respectively, are connected so as to form an optical transmission line,it is possible to construct an optical transmission line whose averagechromatic dispersion and splice loss are so small that it is suitablyused in a wavelength division multiplexing transmission system.

Preferably, heat treatment is carried out for thermal diffusion ofdopants after fusion splice. This heat treatment can further lower thesplice loss. A preferred condition for this heat treatment comprises aheating range with a heating length of at least 1 mm but less than 10 mmcentered at the fusion-splice portion, and a maximum heating temperatureof at least 1300° C. but less than 1800° C. The heating temperature isselected within a temperature range in which the optical fibers 1, 2 arenot deformed while the dopants can thermally diffuse.

The optical transmission line 6 of a conventional product shown in FIG.1B, by contrast, uses an optical fiber 3 as a single-mode optical fiber.As indicated by the refractive index profile shown in FIG. 2C, thisoptical fiber 3 comprises, successively from its center, a core region31 having a maximum refractive index n₃₁ and an outside diameter 2 a ₃,and a cladding region 32 having a refractive index n₃₂, whereas theindividual refractive indices have the relationship of n₃₁>n₃₂ in termsof magnitude. While the optical fiber 3 is based on silica glass, thecore region 31 is doped with GeO₂, and the cladding region 32 issubstantially pure silica glass. As a result, respective viscositiesη₃₁, η₃₂ of the individual regions have the relationship of η₃₁<η₃₂.

When such an optical fibers 3 and the optical fiber 1 arefusion-spliced, these optical fibers deform at their butting portionssince they have a low viscosity at their center regions and both of themare easy to deform, whereby the core regions 11, 31 and inner claddingregion 12 at their connecting portion increase their diameters as shownin FIG. 1B.

In the optical fiber 1 having a smaller mode field diameter, inparticular, even a minute change in the structure of core region altersthe mode field diameter greatly. The splice loss is supposed to haveincreased in the conventional product due to such a reason. By contrast,the optical transmission line 5 of this embodiment has a structure forrestraining the first optical fiber 1 having a smaller mode fielddiameter from deforming at the connecting end face upon fusion splice,so that the occurrence of fluctuation in mode field diameter can besuppressed, whereby splice loss can be prevented from increasing.

Though one having the refractive index profile shown in FIG. 2A isassumed as the first optical fiber 1 having a smaller mode fielddiameter in this embodiment, the refractive index profile of firstoptical fiber 1 is not restricted thereto. More in general, preferableas the first optical fiber 1 is one having at least two cladding regionlayers, in which the outermost cladding region layer has a viscosityhigher than that of the core region. Because of such a configuration,the cladding region does not deform upon fusion splice in the opticalfiber 1, whereby the core region is restrained from deforming uponheating.

Though one having the refractive index profile shown in FIG. 2B isassumed as the second optical fiber 2 having a larger mode fielddiameter in this embodiment, the refractive index profile of secondoptical fiber 2 is not restricted thereto. More in general, preferableas the second optical fiber 2 is one having at least one cladding regionlayer, in which the outermost cladding region layer has a viscositylower than that of the core region. Further preferable as the secondoptical fiber 2 is one in which the outermost cladding region layer hasa viscosity lower than that of the cladding region of first opticalfiber 1. Because of such a configuration, no structural changes occur inthe core region of the optical fiber 2 even when the cladding softensupon fusion splice.

Preferably, the core region 21 of the optical fiber 2 having a largermode field diameter has an outside diameter larger than that of thefirst cladding region 12 of the optical fiber 1 having a smaller modefield diameter. Because of such a configuration, the core region 11 andfirst cladding region 12 heavily influential to structural parameters ofthe optical fiber 1 are in a state as if lidded with the core region 21of optical fiber 2, so as to be surrounded with glass having a highviscosity, whereby their forms can be maintained. Since the forms ofcore region 11 and first cladding region 12 are maintained in theoptical fiber 1, the splice loss becomes lower.

Preferably, in this case, the core region 21 of second optical fiber hasan average viscosity higher than that of the core region 11 and firstcladding region 12 in the first optical fiber. Here, the averageviscosity η_(ave) of the core region 11 and first cladding region 12 inthe optical fiber 1 can be represented by the following expression:$\eta_{ave} = \frac{{\eta_{a} \times S_{a}} + {\eta_{b} \times S_{b}}}{S_{a} + S_{b}}$

where η_(a) is the viscosity of the core portion, η_(b) is the viscosityof the cladding region, and S_(a) and S_(b) are their respectivecross-sectional areas.

Such setting reliably yields the effect of lidding with the core region21 of second optical fiber.

FIG. 6 is a view showing a second embodiment of the optical transmissionline in accordance with the present invention. The optical transmissionline 5 a of this embodiment differs from the optical transmission line 5of the first embodiment in that its second optical fiber 2′ has a doublecladding structure. FIG. 7 is a view for explaining the refractive indexprofile of the second optical fiber 2′. As shown in FIGS. 6 and 7, thesecond optical fiber 2′ comprises, successively from its center, a coreregion 21 having a refractive index n₂₁ and an outside diameter 2 a ₂′,a first cladding region 22 a having a refractive index n₂₂ a and anoutside diameter 2 b ₂, and a second cladding region 22 b having arefractive index n_(22b), whereas the individual refractive indices areset so as to have the relationship of n₂₁>n_(22b)>n_(22a). While theoptical fiber 2 is based on silica glass, the core region 21 issubstantially pure silica glass or doped with about 0.5 wt % to 1.0 wt %of Cl element. Each of the two cladding regions 22 a, 22 b is doped withF element, whereas the first cladding region 22 a has a dopantconcentration higher than that in the second cladding region 22 b. As aresult, each of the viscosities η_(22a), η_(22b) of cladding region 22is smaller than the viscosity η₂₁ of core region 21, and is smaller thanthe viscosity η₁₃ of second cladding 13 of first optical fiber 1 (seeFIG. 4). Effects similar to those of the first embodiment can beachieved in this case as well.

Examples and Comparative Examples

In order to verify the effects of splice loss reduction in the opticaltransmission line in accordance with the present invention, theinventors prepared several kinds of samples and carried out experimentsfor comparing them with conventional optical transmission line samples.The results of experiments will now be explained.

Tables 1 to 4 shown in the following are charts summarizing the opticalfiber structures of individual samples (identified by case numbers),indicating 13 kinds of samples. Here, cases 1 to 3 are structuralexamples of conventional products, i.e., comparative examples, whereascases 4 to 13 are examples of the optical transmission line inaccordance with the present invention.

TABLE 1 Structure of First Optical Fiber Core 1st cladding 2nd claddingoutside outside outside Case diameter diameter diameter No. (μm) (μm)(μm) 1 4.0 8.0 122 2 3.9 7.0 120 3 4.2 8.0 125 4 5 6 7 4.3 9.0 128 8 4.28.0 126 9 4.1 7.0 124 10 4.675 8.5 126 11 5.225 9.5 128 12 5.775 10.5130 13 4.0 8.0 123

TABLE 2 Dopant Concentration and Refractive Index Characteristic ofFirst Optical Fiber 2nd Core 1st cladding dopant cladding dopant Caseconc. Δn1 dopant Δn2 conc. Δn3 No. (wt %) (%) conc. (wt %) (%) (wt %)(%) 1 30.94 1.70 1.365 −0.35 0.00 0.00 2 27.30 1.50 3 26.39 1.45 1.56−0.40 0.20 0.022 4 5 6 7 25.48 1.40 1.17 −0.30 0.455 0.05 8 9 10 27.301.50 1.56 −0.40 11 12 13

As can be seen from Tables 1 and 2, the first optical fiber with asmaller mode field diameter (MFD) in each case has the refractive indexand structure shown in FIG. 2A, in which the core region is silica glassdoped with GeO₂, whereas the fist cladding region is silica glass dopedwith element. The second cladding region is substantially pure silicaglass in cases 1 to 6, and silica glass doped with Cl element in cases 7to 13. The outside diameter of first cladding region is within the rangeof 7 μm to 9 μm in cases 1 to 10 and 13, and exceeds 9 μm and thus islarge in cases 11 and 12. In Table 2, each relative refractive indexdifference is based on the relative refractive index difference of puresilica glass.

TABLE 3 Structure of Second Optical Fiber Core outside Case Struc-diameter Core No. ture (μm) dopant 1 3 7.5 GeO₂ 2 3 6.0 4 2 11.0 none 57.0 6 9.0 Cl 7 11.0 8 12.0 9 9.0 10 9.5 11 8.5 12 7.5 13  2‘ 12.0

TABLE 4 Dopant Concentration and Refractive Index Characteristic ofSecond Optical Fiber 2nd Core 1st cladding dopant cladding dopant Caseconc. Δn1 dopant Δn2 conc. Δn3 No. (wt %) (%) conc. (wt %) (%) (wt %)(%) 1 6.188 0.34 0.0 0.0 — — 2 3 10.01 0.55 0.195 −0.05 4 0.0 0.0 1.248−0.32 5 6 0.455 0.05 0.975 −0.25 7 8 9 0.637 0.07 1.092 −0.28 11 12 130.78 −0.2

The numbers listed in the column of structure in Table 3 indicate whichstructures of optical fibers shown in FIGS. 1A, 1B, and 6 are used asthe second optical fiber. Namely, employed in cases 1 to 3 are thosehaving the refractive index profile shown in FIG. 2C, in which the coreregion is silica glass doped with GeO₂, whereas the cladding region issubstantially pure silica glass (in cases 1 and 2) or silica glass dopedwith F element (in case 3). Employed as the second optical fiber incases 4 to 12 are those having the refractive index profile shown inFIG. 2B, in which the core region is substantially pure silica glass (incases 4 and 5) or silica glass doped with Cl element (in cases 6 to 12),whereas the cladding region is silica glass doped with F element. Incase 13, the second optical fiber is one having the refractive indexprofile shown in FIG. 7, in which the core region is silica glass dopedwith Cl element, whereas the first and second cladding regions are madeof silica glass doped with F element. The outside diameter of coreregion of second optical fiber is within the range of 6.0 to 12.0 μm ineach case. The outside diameter of second optical fiber is 125 μm ineach case, whereas the outside diameter of first cladding in the secondoptical fiber employed in case 13 is set to 50 μm.

Table 5 shows the respective mode field diameters (MFD) of first andsecond optical fibers, respective total average viscosities η₁, η₂thereof, average viscosity η_(1c) of the core region and first claddingregion of first optical fiber, viscosity η_(2c) of the core region ofsecond optical fiber, and results of comparison of splice losses afterfusion and after the above-mentioned predetermined heat treatment whenthese optical fibers are fusion-spliced together are compared with eachother.

TABLE 5 Comparison of Characteristics of First and Second Optical Fibersand Comparison of Splice Loss splice loss after 1st optical fiber 2ndoptical fiber fusion after Case MFD η₁ η_(1c) MFD η₂ η_(2c) spliceheating No. (μm) (×10⁸P) (μm) (×10⁸P) (dB) 1 4.5 30 2.08 10.3 30 1.5451.5 1.0 2 5.0 1.91 1.6 0.9 3 5.1 17 1.52 8.2 18 0.3178 1.0 0.6 4 11.51.3 30.0 0.5 0.2 5 10.5 1.1 30.0 0.8 0.3 6 11.1 2.2 10.36 0.6 0.2 7 4.98.8 2.84 11.7 2.3 0.4 0.15 8 5.2 2.67 12.1 0.35 0.11 9 5.2 2.42 10.3 1.67.958 0.35 0.12 10 5.0 2.57 10.5 0.4 0.15 11 4.8 10.2 0.45 0.3 12 4.710.0 0.55 0.4 13 4.9 1.57 12.0 3.4 0.70 0.2

A necessary condition in the optical transmission line in accordancewith the present invention is that the optical fiber with a smaller MFDhas an average viscosity higher than that of the optical fiber having alarger MFD; cases 4 to 13, which are examples, satisfy this condition ofη₁>η₂. The splice loss after fusion splice was 1.0 dB or greater incases 1 to 3 which are comparative examples, and was within the range of0.35 to 0.8 dB in cases 4 to 13 which are examples. In each case, thesplice loss after heat treatment was smaller than that after fusionsplice. Though the splice loss after heat treatment was still 0.6 dB orgreater in cases 1 to 3 which are comparative examples, it was 0.4 dB orless in cases 4 to 13 which are examples and within the range of 0.11 to0.3 dB in examples excluding case 12.

As in the foregoing, while the splice loss after fusion splice was about1.0 to 2.0 dB in the prior art, it was allowed to decrease to about 0.35to 0.8 dB by use of the optical fiber connecting method in accordancewith this embodiment. Also, while the splice loss after heat treatmentwas about 0.3 to 0.6 dB in the prior art, it was allowed to decrease toabout 0.11 to 0.3 dB by use of the optical fiber connecting method inaccordance with this embodiment.

In particular, among those in accordance with this embodiment, cases 4,6 to 10, and 13 in which the core of second optical fiber has an outsidediameter larger than the outside diameter of first cladding yielded asplice loss of 0.3 dB or less which was small. In cases 7 to 10 amongthem, the splice loss was 0.15 dB or less and was particularly small.FIG. 8 is a graph plotting the splice loss after heat treatment withrespect to the ratio a₂/b₁ of the outside diameter 2 a ₂ of core regionin the second optical fiber to the outside diameter 2 b ₁ of firstcladding in the first optical fiber. This graph indicates it preferableto set a₂/b₁ to at least 1.05, i.e., set the outside diameter of coreregion in the second optical fiber to at least 1.05 times that of thefirst cladding region in the first optical fiber, since the splice lossafter heat treatment can be suppressed to 0.2 dB or less thereby. Thisis because of the fact that, in such setting, the core and innercladding region of first optical fiber, which are easier to deform uponfusion splice, are covered with the harder core portion of secondoptical fiber, whereby their deforming is suppressed. In the case wherethe first optical fiber has a structure made of four or more layers, itis preferred that the core diameter of second optical fiber be set tothe inside diameter of outermost layer of first optical fiber orgreater.

FIG. 9 is a graph plotting the splice loss of each sample after heattreatment with respect to the ratio η_(1c)/η_(2c) of the averageviscosity η_(1c) of the core and first cladding region of first opticalfiber and the viscosity η_(2c) of core region of second optical fiber.It has been seen that, if η 1c/η_(2c) is 1/3 or less, i.e., η_(2c) is atleast three times as much as η_(1c), then the splice loss after heattreatment becomes 0.4 dB or less, whereby favorable characteristics canbe exhibited.

Without being restricted to the above-mentioned embodiments, the presentinvention can be modified in various manners. Though the above-mentionedembodiments explain an optical transmission line in which a single-modeoptical fiber (second optical fiber) having a zero-dispersion wavelengthin a 1.3-μm wavelength band and a positive chromatic dispersion at awavelength of 1.55 μm and a dispersion-compensating optical fiber (firstoptical fiber) having a negative chromatic dispersion at a wavelength of1.55 μm are fusion-spliced, so as to compensate for chromaticdispersion, the present invention is not restricted thereto. Forexample, the present invention is also suitably employed for restrainingtransmission characteristics from deteriorating due to four-wave mixingand chromatic dispersion in an optical transmission line in whichpositive and negative dispersion optical fibers respectively havingpositive and negative chromatic dispersions at a predeterminedwavelength are alternately fusion-spliced.

INDUSTRIAL APPLICABILITY

The present invention is suitably applicable to an optical transmissionline including a portion in which optical fibers having structures andcharacteristics di different from each other are fusion-spliced forsuppressing chromatic dispersion and restraining transmissioncharacteristics from deteriorating due to four-wave mixing and chromaticdispersion.

What is claimed is:
 1. An optical transmission line including afusion-spliced portion formed by fusion-splicing optical fibers havingstructures different from each other, wherein the fusion spliced portioncomprises: a first optical fiber having a mode field diameter, ageometric center, an outermost layer, and an average viscosity; and asecond optical fiber fusion-spliced, having a mode field diametergreater than the mode field diameter of the first optical fiber, ageometric center, an outermost layer, and an average viscosity, whereinthe average viscosity from the geometric center to the outermost layerin said first optical fiber is greater than the average viscosity fromthe geometric center to the outermost layer in said second opticalfiber.
 2. The optical transmission line according to claim 1, wherein,after said first and second optical fibers are fusion-spliced, saidoptical transmission line is heat-treated at the highest heatingtemperature of at least 1300° C. but not exceeding 1800° C. within arange having a length of at least 1 mm but less than 10 mm centered atsaid fusion-splice portion.
 3. The optical transmission line accordingto claim 1, wherein said first optical fiber has at least two claddingregion layers surrounding a core region, and the average viscosity ofthe outermost cladding region layer is greater than the averageviscosity of said core region, and wherein the outermost cladding regionlayer of said first optical coincides with the outermost layer of saidfirst optical fiber.
 4. The optical transmission line according to claim3, wherein said core region of said first optical fiber is doped withGeO₂ at a dopant concentration of at least 18 wt %, the first claddingregion is doped with F element, and the outermost cladding region layeris doped with Cl element.
 5. The optical transmission line according toclaim 3, wherein said core region of said second optical fiber has anoutside diameter greater than the inside diameter of said outermostcladding region layer in said first optical fiber.
 6. The opticaltransmission line according to claim 5, wherein a part of claddingregion in said second optical fiber is doped with F element, said coreregion thereof has an outside diameter of at least 1.05 times the insidediameter of said outermost layer region in said first optical fiber. 7.The optical transmission line according to claim 5, wherein a part ofcladding region in said second optical fiber is doped with F element,said core region thereof has a viscosity greater than three times theaverage viscosity of a region inside said outermost cladding regionlayer in said first optical fiber.
 8. The optical transmission lineaccording to claim 1, wherein said second optical fiber has at least onecladding region layer surrounding a core region, and the averageviscosity of the outermost cladding region layer in said second opticalfiber is lower than the average viscosity of said core region in saidsecond optical fiber and lower than the average viscosity of theoutermost cladding region layer in said first optical fiber.
 9. Theoptical transmission line according to claim 1, wherein said secondoptical fiber has a core region doped with Cl element and a claddingregion doped with F element.
 10. The optical transmission line accordingto claim 9, wherein said second optical fiber has two cladding regionlayers, the outer cladding region layer being doped with F element by anamount smaller than that in the inner cladding region layer, and theouter cladding region layer of said second optical fiber coincides withthe outermost layer of said second optical fiber.
 11. The opticaltransmission line according to claim 1, wherein said first and secondoptical fibers have unlike sign chromatic dispersions.